1. Field of the Invention
The present invention generally relates to a molding method for producing an optical glass element such as a lens and a prism. That is, the present invention particularly relates to a molding method in which thermally softened optical glass is hot-pressed by a mold coated with a thin film of a noble metal alloy and molded into the form of an optical glass element having high surface quality and high surface accuracy. Further, the present invention particularly relates to a mold used in such a molding method.
2. Description of the Prior Art
Recently, an optical glass element, particularly, an optical glass lens tends to be made to have an aspherical shape so as to accomplish both the simplification of the lens construction and the minimization in weight of the lens portion in optical devices at the same time. In producing such an aspherical lens, there are some problems as to a working method and mass producibility in the conventional optical polishing method, and wherein a method that optical glass is thermally pressed and ultraprecisely molded (a direct press-molding method) is expected to be full of promise.
This direct press-molding method is such that a block of optical glass is thermally pressed on an aspherical-shaped mold which has been finished to have a desired surface quality and desired surface accuracy as shown in FIG. 1, or a block of optical glass which has been heated is hotpressed, thereby obtaining an optical glass element.
The optical glass element produced by such a direct press-molding method as described above must be superior in optical properties to an optical glass element produced by the conventional optical polishing method. Particularly, in the case of the aspherical lens, extremely high surface accuracy after molding is required. Therefore, the mold used in the direct press-molding method must have such characteristics that the chemical reaction against the optical glass is minimum under the condition of high temperature that the press-molding surface thereof hardly suffers damage such as a scar or the like and that the mold is improved in heat shock resistance against rapid heating and cooling.
As the mold used in the direct press-molding method, a mold using non-oxides such as silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4) or the like, and a mold in which carbon having high density is coated with silicon carbide (SiC) or the like, have been examined.
The non-oxides such as SiC, Si.sub.3 N.sub.4, or the like, however, have extremely high hardness and therefore it is very difficult to highly accurately work such a material into a spherical or aspherical shape. Further, since such a material is produced by sintering, a material added as a sintering accellator thereof may easily react with the optical glass, so that a molding operation for producing a highly accurate optical glass element can not be performed. Also, the mold in which carbon having high density is coated with an SiC firm may easily react with the optical glass containing lead oxide and alkali elements in large quantities because the coating film is made of beta silicon carbide (.beta.-SiC) and therefore the mold is not suitable for producing such a highly accurate optical glass element.
Now, it is known that the noble metal alloy made for the bulk of platinum or the like is used as a material of an optical glass melting crucible. A mechanical property of the bulk of noble metal alloy, for example, strength or hardness, is inferior to that of other materials. Therefore, it is difficult to say that the properties of the noble metal alloy such as being reaction proof against the optical glass and aving oxidation resistance are sufficiently practically used in producing optical glass elements.
Generally, it is known that various physical properties of a thin film act differently from that of the bulk material and therefore also a mechanical property of the thin film is different from that of the bulk material. Specifically, the mechanical property of the thin film is superior to that of the bulk material, that is, the mechanical property of a thin film of the noble metal alloy is superior to that of the bulk of noble metal alloy.
Thus, a noble metal alloy having superior reaction proof against an optical glass and superior oxidation resistance, is formed into a thin film and a mold for producing an optical glass element is coated with such a thin film to thereby improve the mold in its mechanical property.